Re: [Discuss-gnuradio] HDLC framer

[Nick Foster]

I wrote the HDLC framer to use PMT blobs (specifically, PMT pairs with a length field and a blob) because it seemed that was the convention we were settling upon in GR to handle binary messages. I'm unaware if there was any formal specification or agreement to this format.

To clarify, the missing feature is just dealing with blobs that aren't a multiple of 8 bits in length. To do so would probably require changing the length field to represent nbits instead of nbytes, and using a padding convention (msbit or lsbit) to determine which bits in the last byte of the blob should be included.

I haven't seen too many (len%8) HDLC frames out there so I never got down to putting that in there. The deframer would also have to be modified, specifically crc_ccitt().

--n

On Thu, Dec 4, 2014 at 8:25 AM, Marcus Müller <address@hidden> wrote:

Hi Daniel,
the HDLC framer can only deal with messages containing PMTs that are
"blobs", ie. u8vectors.
In fact, that's a missing feature:
        pmt::pmt_t len(pmt::car(msg)); //TODO for non-mult-8 nbits
        pmt::pmt_t blob(pmt::cdr(msg));
        if(!pmt::is_blob(blob))
            throw std::runtime_error("HDLC framer: PMT must be blob");

Instead of making a pmt.intern("String"), you will need to have a
u8vector. It's a bit ugly to generate that in single line:

pmt.pmt_to_python.numpy_to_uvector(numpy.array([ord(c) for c in "Hello
World"], numpy.uint8))

you might need to add an "import block" that does an "import numpy".

Greetings,
Marcus

On 12/04/2014 11:52 AM, Daniel Batista wrote:
> Thank you Marcus,
> I try the following:
> MessageStrobe("Hello World") ----> HDCL framer ---->HDLC deframer ----> Message Debug
> and I get the following error message:thread[thread-per-block[2]: <block hdlc_framer_pb (1)>]: pmt_car: wrong_type : Hello World!
> What did I do wrong?
> Daniel

>
>
>
>      On Wednesday, December 3, 2014 10:55 AM, Marcus Müller <address@hidden> wrote:
>
>
>  Hi Daniel,
>
> PMT is short for "polymorphic type" [1].
> GNU Radio uses PMTs for stream tags and, more importantly here, for
> asynchronous message ports.
> So the idea is that you have a source of messages[2], sending your
> data to the HDLC framer, which frames it and generates output items.
> If you want some illustrated guide through these concept, I'd
> recommend the new guided tutorials on gnuradio.org[3] numbers 1 through 5.
>
> Greetings, and happy hacking,
> Marcus
>
> [1] http://gnuradio.org/doc/doxygen/page_pmt.html
> [2] http://gnuradio.org/doc/doxygen/page_msg_passing.html
> [3] http://gnuradio.org/redmine/projects/gnuradio/wiki/Guided_Tutorials
>
> On 12/02/2014 10:47 AM, Daniel Batista wrote:
>> Hi, I read in the documentation that HDLC framer takes in PMT
>> binary blobs and outputs HDLC frames as unpacked bits.I don't
>> undestand what are the PMT binary blobs.How can I put in an HDLC
>> frame a "Hello World" message? ThanksDaniel
>>
>>
>>
>>
>> _______________________________________________ Discuss-gnuradio
>> mailing list address@hidden
>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>>
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>
>
>


_______________________________________________
Discuss-gnuradio mailing list
address@hidden
https://lists.gnu.org/mailman/listinfo/discuss-gnuradio